When a mobile radiotelephone is handed off from one radiotelephone cell to another, it typically must change from the frequency that was used in the old cell to a new frequency that will be used in the new cell. This hand-off takes place while the radiotelephone is in a call, and therefore must be done quickly enough to avoid a drop in the audio signal. This drop will cause a gap in conversation in the call. The U.S. Digital Cellular specifications require that the frequency difference between the receiver and the incoming signal be within .+-.200 Hz. The receiver, therefore, must quickly lock onto the new frequency to stay within these limits.
A phase locked loop (PLL) is typically used in the radiotelephone to change from one frequency to another. PLLs are discussed in A. Blanchard, Phase Locked Loops: Application to Coherent Receiver Design 281-292 (1976) and F. Gardner, Phaselock Techniques (1979). One method for decreasing the PLLs lock time is to use an adaptive bandwidth filter to narrow the signal's bandwidth when the phase error crosses the zero axis. The problem with this method, however, is that it takes a relatively large amount of time for the voltage controlling the voltage controlled oscillator (VCO), the warp voltage, to reach zero. There is a resulting need for a method to greatly reduce the time for a PLL to lock onto a frequency.